A universal joint of this sort is also known as a gimbal, and defines an isostatic constraint for the engine to compensate, on the one hand, for movements caused by thermal expansion, and, on the other, for relatively small movements transmitted to the engine by the reduction gear and caused by loads exerted on the reduction gear by the main rotor of the helicopter.
The spider of known non-rotating universal joints is defined by a ring with a space in the middle for a coaxial shaft, which rotates to transmit power from the engine to the reduction gear. The ring is connected to a first pair of diametrically opposite arms fixed with respect to the engine casing, and to a second pair of arms offset 90° with respect to the first pair and fixed with respect to the reduction gear casing.
The ring and the ends of the arms are connected by respective bolts extending radially with respect to the axis of the ring and the rotary transmission shaft, and each of which comprises a screw with a head resting on the outer surface of the relative arm; and a nut screwed to the end of the screw shank and resting on an inner surface of the ring.
The ring normally exerts shear stress on the bolts and bending stress on the arms, which are difficult to quantify. In the known solutions described, the shear stress on the screw shanks and the bending stress on the arms are relatively severe, on account of the screws projecting from the arms; and known solutions are subject to fretting, i.e. wear caused by the relatively moving contact surfaces rubbing against each other, particularly the surfaces of the screw shanks against the inner surfaces of the holes in the ring.
Wear and severe shear and bending stress result in failure of the screws and arms, so that the bolts and gimbal as a whole must be reinforced at the design stage, thus resulting in increased weight.
To minimize shear stress on the shank of each screw and bending stress on the arms without increasing weight, the arms may be designed with end forks engaged by respective portions, with holes, of the spider ring; and each of the four screws fits through the respective fork, i.e. is supported at two points spaced apart. By eliminating projection of the screws, this type of assembly reduces bending and, therefore, shear stress exerted by the ring on the screws. One solution of this type is described as known art in Patent EP1539573.
A need is felt to further improve the above known solutions, to reduce wear caused by fretting and shock, and to minimize the bulk thereof.